KR102518675B1 - High strength cold-rolled steel sheet with excellent formability and mathod of manufacturing the same - Google Patents

Abstract

A high-strength cold-rolled steel sheet with excellent formability and a manufacturing method thereof are disclosed.
The high-strength cold-rolled steel sheet according to the present invention contains C: 0.05-0.12%, Si: 0.5% or less, Mn: 2.0-3.0%, Cr: 0.3-1.2%, Ti: 0.02-0.08%, Nb: 0.01-0.06% by weight. %, B: contains 0.001 to 0.005%, is composed of the remaining Fe and unavoidable impurities, has a transformation structure containing at least one of bainite and martensite, and a microstructure including ferrite, and the transformation of the microstructure The structure fraction is 60 to 90% in area ratio, the ferrite fraction is 10 to 40% in area ratio, and the average grain diameter of the transformed structure is 3 μm or less.

Description

High-strength cold-rolled steel sheet with excellent formability and method for manufacturing the same

The present invention relates to a high-strength cold-rolled steel sheet having excellent formability.

In addition, the present invention relates to a method for manufacturing a high-strength cold-rolled steel sheet having excellent formability.

Recently, steel sheets for automobiles are required to have higher strength in order to improve fuel efficiency and durability. In particular, with the recent spread of regulations on the impact stability of automobiles, high-strength steel sheets with excellent yield strength are used for structural members such as members, seat rails, and pillars to improve the impact resistance of automobile bodies. The higher the yield strength to tensile strength, that is, the higher the yield ratio, the more advantageous the impact energy absorption capacity, and structural members require a high yield ratio.

In addition, recently, in order to further improve the safety of passengers in a collision, high-strength and lightweight seat parts of automobiles are progressing at the same time. These parts are manufactured by two methods: roll forming as well as press forming. The seat part is a part that connects the passenger and the vehicle body, and must be supported with high stress to prevent the passenger from being thrown out in the event of a collision. This requires high yield strength and high yield ratio. In addition, as most of the parts to be machined require expansion flange properties, the application of steel materials with excellent hole expandability is required.

However, since the formability generally decreases as the strength of the steel sheet increases, a steel sheet having high strength and excellent formability is required.

Registered Patent Publication No. 10-1677396 (Announced on November 18, 2016)

An object to be solved by the present invention is to provide a high-strength cold-rolled steel sheet having a tensile strength of 980 MPa or more having excellent formability.

Another problem to be solved by the present invention is to provide a method for manufacturing a high-strength cold-rolled steel sheet having excellent strength and formability.

The objects of the present invention are not limited to the above-mentioned objects, and other objects and advantages of the present invention not mentioned above can be understood by the following description and will be more clearly understood by the examples of the present invention. It will also be readily apparent that the objects and advantages of the present invention may be realized by means of the instrumentalities and combinations indicated in the claims.

High-strength cold-rolled steel sheet according to an embodiment of the present invention for solving the above problems contains C: 0.05 ~ 0.12%, Si: 0.5% or less, Mn: 2.0 ~ 3.0%, Cr: 0.3 ~ 1.2%, Ti: 0.02% by weight ~0.08%, Nb: 0.01~0.06%, B: 0.001~0.005%, composed of the rest of Fe and unavoidable impurities, and a transformation structure containing at least one of bainite and martensite and a microstructure containing ferrite It has a structure, the transformation structure fraction of the microstructure is 60 to 90% in area ratio, the fraction of the ferrite is 10 to 40% in area ratio, and the average particle diameter of the transformation structure is 3 μm or less.

The high-strength cold-rolled steel sheet may further include, in mass%, P: 0.001 to 0.10%, S: 0.01% or less, Sol.Al: 0.01 to 0.10%, and N: 0.01% or less.

The high-strength cold-rolled steel sheet may include martensite having an average grain diameter of 2 µm or less, bainite having an average grain diameter of 3 µm or less, and ferrite having an average grain diameter of 3 µm or less.

The ratio of bainite and ferrite having an average grain diameter of more than 3 μm in the microstructure may be 5% or less in terms of area ratio.

Retained austenite may not be included in the microstructure.

The ferrite may include non-recrystallized ferrite having an area ratio of 25% or less and recrystallized ferrite having an area ratio of 75% or more.

The microstructure may include nano-precipitates of 10 nm or less at a distribution density of 150 pieces/μm ² or more.

The high-strength cold-rolled steel sheet may have tensile strength of 980 MPa or more, elongation of 14% or more, bending workability (R/t) of 1.0 or less, and hole expandability (HER) of 40% or more.

High-strength cold-rolled steel sheet manufacturing method according to an embodiment of the present invention for solving the above problems is C: 0.05 ~ 0.12%, Si: 0.5% or less, Mn: 2.0 ~ 3.0%, Cr: 0.3 ~ 1.2%, Ti in weight % : 0.02 to 0.08%, Nb: 0.01 to 0.06%, B: 0.001 to 0.005%, hot-rolling a steel base material composed of the remaining Fe and unavoidable impurities to prepare a hot-rolled steel sheet; After cold-rolling the hot-rolled steel sheet, an annealing heat treatment step; and overaging the steel sheet subjected to the annealing heat treatment, and is characterized by satisfying Equation 1 below.

[Equation 1]

13≤0.8A-0.4(B-C)≤27

A: Cold rolling reduction (%)

B: Ac3 temperature (℃)

C: annealing heat treatment temperature (℃)

The annealing heat treatment may include maintaining the cold-rolled steel sheet at a temperature that satisfies Equation 1, and cooling the maintained steel sheet to a temperature equal to or less than Ms.

Skin pass rolling of the cold-rolled steel sheet at a reduction ratio of 1% or less may be further included.

A step of forming a zinc plating layer or an alloy plating layer on the surface of the cold-rolled steel sheet may be further included.

According to the present invention, a high-strength cold-rolled steel sheet capable of exhibiting a high tensile strength of 980 MPa or more, an elongation of 14% or more, a HER value (hole expandability) of 40% or more, and an R/t value (bending workability) of 1.0 or less can be provided. can

The high-strength cold-rolled steel sheet according to the present invention is suitable for use in structural members such as members, seat rails, and pillars of automobiles based on excellent strength and formability.

In addition to the effects described above, specific effects of the present invention will be described together while explaining specific details for carrying out the present invention.

1 shows the microstructure of steel specimen 1-1 (inventive steel).
2 shows the distribution of fine precipitates in steel specimen 1-1 (inventive steel).
3 shows the microstructure of steel specimens 1-4 (comparative steel).

Advantages and features of the present invention, and methods of achieving them, will become clear with reference to the detailed description of the following embodiments taken in conjunction with the accompanying drawings. However, the present invention is not limited to the embodiments disclosed below and will be implemented in various forms different from each other, only these embodiments make the disclosure of the present invention complete, and common knowledge in the art to which the present invention pertains. It is provided to completely inform the person who has the scope of the invention, and the present invention is only defined by the scope of the claims. Like reference numbers designate like elements throughout the specification.

Hereinafter, with reference to the accompanying drawings, a high-strength cold-rolled steel sheet having excellent formability according to a preferred embodiment of the present invention and a manufacturing method thereof will be described in detail.

Methods for strengthening steel to increase the strength of steel sheets include solid solution strengthening, precipitation strengthening, strengthening by crystal grain refinement, transformation strengthening, and the like. Among the above methods, strengthening by solid solution strengthening and crystal grain refinement has a disadvantage in that it is difficult to manufacture high-strength steel having a tensile strength of 490 MPa or more. Precipitation strengthening strengthens steel sheets by precipitating carbon and nitrides by adding carbon and nitride-forming elements such as Cu, Nb, Ti, V, etc. to steel, or refining crystal grains through suppression of crystal grain growth by fine precipitates to secure strength. It is a skill. This technology has the advantage of easily obtaining high strength at a low manufacturing cost. However, in the case of precipitation strengthening, the recrystallization temperature is rapidly increased by fine precipitates, and accordingly, in order to secure ductility through sufficient recrystallization, there is a disadvantage in that high-temperature annealing must be performed. In addition, precipitation strengthening has a problem in that it is difficult to obtain high-strength steel having a tensile strength of 600 MPa or more.

On the other hand, transformation-strengthened high-strength steel is ferrite-martensite dual phase steel containing hard martensite in a ferrite base, TRIP (Transformation Induced Plasticity) steel using transformation-induced plasticity of retained austenite, or ferrite and Various types such as CP (Complexed Phase) steel composed of a hard bainite or martensite structure have been developed.

The easiest way to produce martensite is to hold it for a time during which austenite can be sufficiently formed during annealing heat treatment, followed by rapid cooling and tempering. However, the rapid cooling method may lead to deterioration in productivity due to problems such as material variation and shape defects. Accordingly, in the present invention, it was attempted to secure martensite by controlling alloy elements. That is, by adding a certain amount or more of hardenable elements such as Mn and Cr, martensite can be secured even at a low cooling rate. In the case of high alloying element addition, problems such as deterioration of weldability may occur. In the present invention, the carbon content that has the greatest effect on weldability is limited to 0.12% by weight or less.

High-strength cold-rolled steel sheet

The high-strength cold-rolled steel sheet according to the present invention contains C: 0.05-0.12%, Si: 0.5% or less, Mn: 2.0-3.0%, Cr: 0.3-1.2%, Ti: 0.02-0.08%, Nb: 0.01-0.06% by weight. %, B: 0.001 to 0.005%. In addition, the high-strength cold-rolled steel sheet according to the present invention may further include, in mass%, P: 0.001 to 0.10%, S: 0.010% or less, Sol.Al: 0.01 to 0.10%, and N: 0.01% or less. Other than the above components, the remainder consists of Fe and unavoidable impurities.

Hereinafter, the role and content of components included in the high-strength cold-rolled steel sheet according to the present invention will be described.

carbon (C)

Carbon (C) is a very important element added to reinforce metamorphosis. In addition, C promotes high strength and promotes the formation of martensite. However, when the C content exceeds 0.12% by mass, the strength of martensite increases, but the difference in strength from ferrite with a low carbon concentration increases. This difference in strength reduces hole expandability because breakage easily occurs at the interphase interface when stress is applied. In addition, when the C content exceeds 0.12% by mass, weldability deteriorates and the possibility of occurrence of welding defects increases. On the other hand, when the C content is less than 0.05% by mass, it is very difficult to secure the strength of martensite presented in the present invention. Accordingly, in the present invention, the C content was limited to 0.05 to 0.12 mass%.

Silicon (Si)

Silicon (Si) promotes ferrite transformation and increases the carbon content in non-transformed austenite to form a complex structure of ferrite and martensite, thereby hindering the increase in strength of martensite. In addition, silicon not only causes surface scale defects but also deteriorates conversion processability. Accordingly, in the present invention, the addition amount of silicon is limited to 0.5% by mass or less.

Manganese (Mn)

Manganese (Mn) is an element that refines particles without damaging ductility, precipitates sulfur (S) in steel as MnS, prevents hot brittleness due to the generation of FeS, and strengthens steel. In addition, Mn contributes to more easily forming martensite by serving to lower the critical cooling rate at which martensite phase is obtained. If the content of Mn is less than 2.0% by mass, it is difficult to secure the target strength in the present invention, whereas if it exceeds 3.0% by mass, the possibility of problems such as weldability and hot rolling increases. Therefore, in the present invention, the content of Mn is limited to 2.0 to 3.0 mass%, more preferably 2.3 to 2.9 mass%.

Chromium (Cr)

Chromium (Cr) is a component added to improve the hardenability of steel and secure high strength, and in the present invention, it is an element that plays a very important role in forming martensite, a low-temperature transformation phase. When the content of Cr is less than 0.3% by mass, it is difficult to secure the above effects. On the other hand, when the Cr content exceeds 1.2% by mass, the effect is saturated, and cold rolling property deteriorates due to an excessive increase in hot rolling strength. Accordingly, in the present invention, the content of Cr was limited to 0.3 to 1.2 mass%.

Titanium (Ti), Niobium (Nb)

Titanium (Ti) and niobium (Nb) are effective elements for increasing the strength of a steel sheet and refining crystal grains by nano-precipitates. Ti and Nb combine with C to form very fine nano-precipitates. These nano-precipitates serve to strengthen the base structure and reduce the hardness difference between the phases (for example, 1.3 or less). In the present invention, the content of Ti is limited to 0.02 to 0.08%, and the content of Nb is limited to 0.01 to 0.06% by mass. When the content of Ti is less than 0.02% by mass or the content of Nb is less than 0.01% by mass, the distribution density of nano-precipitates does not satisfy the value suggested by the present invention steel. On the other hand, when the content of Ti exceeds 0.08% by mass or the content of Nb exceeds 0.06% by mass, ductility may be greatly reduced due to increased manufacturing cost and excessive precipitates.

Boron (B)

Boron (B) serves to promote the formation of martensite in the process of cooling after annealing heat treatment. When the addition amount of B is less than 0.001% by mass, it is difficult to obtain the above addition effect. On the other hand, if the addition amount of B exceeds 0.005% by mass, excessive cost increase may occur due to excessive formation of ferroalloy. Accordingly, in the present invention, the content of B was limited to 0.001 to 0.005% by mass.

Phosphorus (P)

Phosphorus (P) is a substitutional alloying element having the greatest solid solution strengthening effect and serves to improve in-plane anisotropy and improve strength. If the content of P is less than 0.001% by mass, the effect of its addition is insufficient, so if P is added, the content is preferably 0.001% by mass or more. On the other hand, when the content of P exceeds 0.1% by mass, press formability may deteriorate and brittleness of the steel may occur. Accordingly, in the present invention, the addition amount of P was limited to 0.001 to 0.10 mass%.

Sulfur (S)

Sulfur (S) is an impurity element in steel and is an element that impairs ductility and weldability of steel sheets. If the S content exceeds 0.01% by mass, it is highly likely to impair the ductility and weldability of the steel sheet, so the S content is preferably limited to 0.01% by mass or less.

Soluble Aluminum (Sol.Al)

Soluble aluminum (Sol.Al) combines with oxygen in steel to contribute to deoxidation, and is an effective component for improving martensitic hardenability by distributing carbon in ferrite to austenite together with Si. When the Sol.Al content is less than 0.01% by weight, the addition effect is insufficient. On the other hand, when the content of Sol.Al exceeds 0.1% by weight, not only the effect of the addition is saturated, but also the manufacturing cost may increase. Accordingly, in the present invention, the content of Sol.Al was limited to 0.01 to 0.1% by weight.

Nitrogen (N)

Nitrogen (N) is a component that acts effectively in stabilizing austenite, and if it exceeds 0.01%, the risk of cracking during playing through AlN formation is greatly increased, so it is preferable to limit the upper limit to 0.01%.

The high-strength cold-rolled steel sheet according to the present invention may have a transformation structure including at least one of bainite and martensite and a microstructure including ferrite according to the alloy components and the manufacturing method described below. Martensite is more preferably tempered martensite. The transformation structure fraction of the microstructure may be 60 to 90% in area ratio, the ferrite fraction may be 10 to 40% in area ratio, and the average particle diameter of the transformation structure may be 3 μm or less.

In addition, the high-strength cold-rolled steel sheet according to the present invention may include martensite having an average grain diameter of 2 µm or less, bainite having an average grain diameter of 3 µm or less, and ferrite having an average grain diameter of 3 µm or less.

The ratio of bainite and ferrite having an average grain diameter of more than 3 μm in the microstructure may be 5% or less in terms of area ratio.

Retained austenite may not be included in the microstructure.

The ferrite may be composed of unrecrystallized ferrite having an area ratio of 25% or less, more preferably 15% or less, and recrystallized ferrite having an area ratio of 75% or more.

In addition, the microstructure may include nano-precipitates of 10 nm or less at a distribution density of 150 pieces/μm ² or more.

The high-strength cold-rolled steel sheet according to the present invention may have tensile strength of 980 MPa or more, elongation of 14% or more, bending workability (R/t) of 1.0 or less, and hole expandability (HER) of 40% or more.

In order to improve bending workability and hole expandability, the transformed tissue fraction is required to be 60% or more in terms of area ratio. The fraction was controlled to be 60 to 90% as an area ratio. In addition, in order to improve bending workability, hole expandability, elongation and strength, it is desirable to control the average grain size of the transformed structure as small as possible, and in the present invention, the average grain size of the transformed structure is limited to 3 μm or less. More specifically, the average grain size of martensite was controlled to 2 µm or less, and the average grain size of bainite was controlled to 3 µm or less. In addition, ferrite also preferably has an average grain size of 3 μm or less. In the present invention, the fraction of bainite and ferrite exceeding 3 μm is 5% or less in terms of area ratio.

When the carbon content is as low as 0.12% by mass or less as in the present invention, when an alloy element is added in consideration of weldability and hot-rolled strength, a limit occurs in increasing the strength of the martensite produced. That is, if sufficient carbon is not included in martensite, there is a limit to increase in strength. In the present invention, it was attempted to improve the strength of the structure by using fine precipitates. That is, according to the study ^of the present inventor, it is desirable to make the size of the precipitate as small as possible in order to improve the strength of the microstructure. The strength of was increased, and it was possible to manufacture a high-strength cold-rolled steel sheet having a bending workability (R/t) of 1.0 or less and a hole expandability (HER) of 40% or more.

High-strength cold-rolled steel sheet manufacturing method

The high-strength cold-rolled steel sheet according to the present invention can be manufactured through the following method.

First, a steel base material such as a steel slab having the above composition is hot-rolled and then wound to manufacture a hot-rolled steel sheet. The steel base material can be reheated for about 1 to 3 hours at a temperature of about 1100 to 1300 ° C before hot rolling. Hot rolling may be performed at a finishing temperature of Ar3 or higher, for example, at a finishing temperature of about Ar3 to Ar3 + 50 ° C. When hot rolling is performed at a temperature lower than Ar3, the hot deformation resistance is likely to increase rapidly, and the top, tail, and edge of the hot-rolled coil become single-phase regions, resulting in an increase in in-plane anisotropy and formability. It can be. However, if the hot rolling finish temperature exceeds Ar3+50°C, thick oxide scale may occur and microstructure coarsening of the steel sheet may occur.

Winding may be performed at 500 to 750 °C. If the coiling temperature is too low, such as less than 500° C., excessive martensite or bainite is generated, resulting in an excessive increase in strength of the hot-rolled steel sheet, which may cause manufacturing problems such as shape defects due to load during cold rolling. On the other hand, when the coiling temperature exceeds 750 ° C., pickling properties may deteriorate due to an increase in surface scale.

Thereafter, after the hot-rolled steel sheet is cold-rolled, annealing heat treatment is performed. Pickling may be performed prior to cold rolling. Annealing heat treatment may be performed in a continuous annealing manner.

The annealing heat treatment may include a holding step and a cooling step of maintaining the annealing heat treatment temperature at about 800 to 820 ° C. The cooling step may include primary cooling and secondary cooling. The first cooling step is a step for suppressing ferrite transformation, and the first cooling may be performed by cooling to about 650 to 700° C. at an average cooling rate of about 1 to 10° C./sec. In the secondary cooling, the austenite secured in the annealing heat treatment is transformed into a transformed structure through rapid cooling to form a transformed structure with an area ratio of 60% or more as suggested in the present invention. The secondary cooling is performed at an average cooling rate of about 5 to 20° C./sec below the Ms temperature, for example, to (Ms-100) to Ms° C. On the other hand, the secondary cooling end temperature is a very important temperature condition for securing high yield strength and high hole expandability as well as securing the shape of the coil in the width and length directions. If the secondary cooling end temperature is too low, less than Ms-100 ° C, the yield strength and tensile strength increase simultaneously due to the excessive increase in the martensite fraction during the overaging treatment, and the ductility is greatly deteriorated. In particular, deterioration in shape due to rapid cooling may occur, which may cause problems such as deterioration of workability during machining of automobile parts. On the other hand, if the secondary cooling end temperature is higher than the Ms temperature, austenite generated during annealing may not be transformed into martensite, and granular bainite, a high-temperature transformation phase, may be formed, resulting in deterioration of yield strength. there is. The formation of such a high-temperature transformation phase may be accompanied by a decrease in yield ratio and deterioration in hole expandability.

Thereafter, the steel sheet subjected to the annealing heat treatment is subjected to overaging treatment. The overaging treatment may be performed for about 100 to 800 seconds at the cooling end temperature, for example. Through overaging, martensite produced through cooling is converted into tempered martensite. It is most preferable that all martensite is converted to tempered martensite by overaging. However, martensite and tempered martensite may coexist after overaging due to overaging conditions.

At this time, in the method for manufacturing a high-strength cold-rolled steel sheet according to the present invention, the relationship between the cold rolling reduction and the annealing heat treatment temperature satisfies Equation 1 below.

[Equation 1]

13≤0.8A-0.4(B-C)≤27

A: Cold rolling reduction (%)

B: Ac3 temperature (℃)

C: annealing heat treatment temperature (℃)

The Ac3 may be determined as, for example, 910-203C ^1/2 -15.2Ni+44.7Si+104V+31.5Mo+13.1W.

In Equation 1, the result value increases as the cold reduction ratio increases and the annealing heat treatment temperature increases, and the result value decreases as the cold reduction ratio decreases and the annealing heat treatment temperature decreases. The inventors of the present invention conducted numerous experiments on steel sheets satisfying the composition proposed in the present invention in order to obtain high elongation while securing strength of the steel sheet. , It was found that when these factors satisfy Equation 1 above, it is possible to secure 14% or more of the drawing wool while having a bending workability (R / t) of 1.0 or less and a hole expansion ratio (HER, Hole Expansion Ratio) of at least 40% or more. . When the resultant value of Equation 1 is less than 13 or greater than 27, at least one of elongation, hole expandability, and bending workability does not meet the target standard.

If the Ac3 temperature is about 865 to 875 ° C and the annealing heat treatment temperature is about 800 ° C, it is preferable that cold rolling is performed at about 40 to 60% in order to satisfy Equation 1.

Meanwhile, the manufactured cold-rolled steel sheet may be further subjected to skin pass rolling at a reduction ratio of 1% or less, preferably 0.1 to 1.0%. Through this, the yield strength of about 50 to 100 MPa can be increased without increasing the tensile strength. On the other hand, if the reduction ratio of skin pass rolling exceeds 1.0%, the yield strength may be excessively increased and formability may be deteriorated, and operability may be greatly unstable due to high elongation work.

A step of forming a zinc plating layer such as a hot-dip galvanized layer or an electro-galvanized layer or an alloy plating layer such as an alloyed hot-dip galvanized layer may be further included on the surface of the manufactured cold-rolled steel sheet.

Example

Hereinafter, the configuration and operation of the present invention will be described in more detail through preferred embodiments of the present invention. However, this is presented as a preferred example of the present invention and cannot be construed as limiting the present invention by this in any sense.

Contents not described herein can be technically inferred by those skilled in the art, so descriptions thereof will be omitted.

1. Preparation of specimens

A steel slab composed as shown in Table 1 below was melted in vacuum, heated in a heating furnace at a reheating temperature of 1200 ° C for 1 hour, hot rolled at a finishing temperature (FDT) of 880 ° C, and then coiled at a temperature of 600 ° C (CT). It was wound in to prepare a hot-rolled steel sheet. The prepared hot-rolled steel sheet was pickled and cold-rolled at the cold reduction ratio (%) shown in Table 2. The cold-rolled steel sheet was continuously annealed under the conditions of the annealing heat treatment temperature (SS) in Table 2 and then cooled in two stages (1st cooling rate: 2°C/sec, 1st cooling end temperature: 650°C, 2nd cooling rate: 15°C). /sec, secondary cooling end temperature: 350 ° C) was performed. Thereafter, skin pass rolling was finally performed at a rolling reduction of 0.2%.

[Table 1] (unit: mass %)

[Table 2]

Table 3 shows the characteristics of each specimen.

The characteristics related to the microstructure of each specimen, that is, transformation fraction, martensite (M) average grain diameter, bainite (B) average grain diameter, non-recrystallized ferrite fraction, and nanoprecipitate density, were analyzed using a scanning electron microscope.

Tensile strength (TS), yield strength (YS) and elongation (T-El) of each specimen were measured using a JIS No. 5 tensile test piece.

The hole expandability (HER) of each specimen was calculated according to Equation 2 below, when _Do is the initial hole diameter (mm) and D _h is the hole diameter after fracture (mm).

[Equation 2]

HER(%)= (D _h -D _o )/D _o ×100

The bending workability (R/t) of each specimen was evaluated by a 90 degree bending test. R is the bending radius of a punch that does not generate cracks after a 90-degree bending test, and t is the thickness (mm) of the specimen.

[Table 3]

In the case of steel specimens 1-1, 1-3, 2-1, 3-1, and 4-1 (inventive steel) satisfying the composition range (Table 1) and manufacturing conditions (Table 2) presented in the present invention, transformation The phase fraction was at least 60% or more in terms of area ratio, the average grain size of martensite was 1.9 µm or less, and the average grain size of bainite was 2.8 µm or less. In addition, in the case of steel specimens 1-1, 1-3, 2-1, 3-1, and 4-1 (inventive steel), unrecrystallized ferrite among ferrites satisfied the area fraction of 25% or less. On the other hand, nano precipitates of 10 nm or less satisfied 150/μm ² or more as suggested in the present invention.

As described above, steel specimens 1-1, 1-3, 2-1, 3-1, and 4-1 (inventive steel) satisfying the characteristics presented in the present invention are distributed in the range of elongation of 15 to 17%, and bending It was found that workability (R/t) was 0 to 0.5 and hole expandability (HER) was 40% to 60%, indicating excellent elongation and stretch flangeability.

1 shows the microstructure of steel specimen 1-1 (inventive steel), and as can be seen in FIG. 1, the martensite/bainite structure has an area ratio of 50% or more, and the average martensite grain diameter is 2 μm. or less, and the average particle diameter of bainite is 3 μm or less. In addition, in the results showing the fine precipitates in FIG. 2, a large amount of very fine nano-precipitates of 10 nm or less, such as TiC and NbC, were distributed. This may be advantageous in securing a bending workability (R/t) of 1.0 or less, an elongation of 14% or more, and a HER value of 40% or more.

On the other hand, steel specimens 1-2, 1-4, 1-5, which do not satisfy the composition range of the steel presented in the present invention, or whose annealing heat treatment conditions do not satisfy the conditions presented in the present invention even if the composition range of the steel is satisfied, In the case of 2-2, 3-2, 4-2, 5, 6, and 7, the material characteristics required by the present invention were not satisfied.

In the case of steel specimens 1-2 and 1-5 (comparative steel), the annealing heat treatment temperature is very high as 850 ° C, and does not satisfy Equation 1 presented in the present invention, so it is formed during cooling due to the increase in austenite grain size due to high temperature annealing heat treatment The average particle diameter of martensite to be formed was more than 2.0 μm, and the average particle diameter of bainite also exceeded 3.0 μm. In addition, 90% or more of the austenite generated during the high-temperature annealing heat treatment was transformed into martensite, so the elongation did not reach the target value.

Steel specimens 1-4, 2-2, 3-2, and 4-2 (comparative steels) had cold reduction rates of 40 to 45%, which did not satisfy Equation 1 presented in the present invention, as shown in FIG. Likewise, the unrecrystallized ferrite fraction was about 30% or more in area ratio, and the elongation did not reach the target value.

Steel specimen 5 (comparative steel) did not meet the carbon (C) content of 0.05% by mass or more specified in the present invention. This low carbon content serves to reduce the strength of martensite generated in the quenching process after annealing heat treatment, and it is difficult to secure a tensile strength of 980 MPa or more.

Steel specimen 6 (comparative steel) had a very high silicon (Si) content. Si is a ferrite-forming element, and when its added amount increases, ferrite production is promoted during cooling. Steel specimen 6 did not satisfy the 60% criterion presented in the present invention steel because the fraction of the transformation structure generated due to the high Si addition was only 55% in area ratio, and the hole expandability (HER) and bending workability (R /t) was not good.

Steel specimen 7 (comparative steel) had a carbon (C) content exceeding 0.12% by mass suggested in the present invention. This high carbon content contributes to increasing the strength of martensite produced in the rapid cooling process after annealing heat treatment. However, all martensite generated during the overaging treatment after quenching is not tempered and remains in a large amount in a lath form. As a result, it is difficult to secure an elongation rate of 14% or more, which is the target. In addition, in the case of tempered martensite generated by overaging treatment, the strength is reduced due to the precipitation of carbon, but the remaining lath-type martensite that is not tempered is a very stable martensite and has very high strength due to the added carbon. have Therefore, when the carbon content exceeds the components presented in the present invention, the hole expandability (HER) and bending workability (R /t) does not satisfy the criteria presented in the present invention.

As described above, the present invention has been described with reference to the drawings illustrated, but the present invention is not limited by the embodiments and drawings disclosed in this specification, and various modifications are made by those skilled in the art within the scope of the technical idea of the present invention. It is obvious that variations can be made. In addition, although the operational effects according to the configuration of the present invention have not been explicitly described and described while describing the embodiments of the present invention, it is natural that the effects predictable by the corresponding configuration should also be recognized.

Claims (13)

C: 0.06-0.12%, Si: 0.5% or less, Mn: 2.0-3.0%, Cr: 0.3-1.2%, Ti: 0.02-0.08%, Nb: 0.01-0.06%, B: 0.001-0.005% by weight % Including, consisting of the remaining Fe and unavoidable impurities,
It has a transformation structure including at least one of bainite and martensite and a microstructure including ferrite, the transformation structure fraction of the microstructure is 60 to 90% in area ratio, and the ferrite fraction is 10 in area ratio ~ 40%, the average grain size of the transformed structure is 3㎛ or less, and the ferrite is composed of non-recrystallized ferrite of 25% or less (excluding 0%) in area ratio and recrystallized ferrite of 75% or more in area ratio. High-strength cold-rolled steel sheet.
According to claim 1,
The high-strength cold-rolled steel sheet, in terms of mass%, P: 0.001 ~ 0.10%, S: 0.01% or less, Sol.Al: 0.01 ~ 0.10% and N: high-strength cold-rolled steel sheet, characterized in that it further comprises 0.01% or less.
According to claim 1 or 2,
The high-strength cold-rolled steel sheet is high-strength cold-rolled steel sheet, characterized in that it comprises martensite having an average grain diameter of 2㎛ or less, bainite having an average grain diameter of 3㎛ or less, and ferrite having an average grain diameter of 3㎛ or less.
According to claim 3,
High-strength cold-rolled steel sheet, characterized in that the ratio of bainite and ferrite having an average grain diameter of more than 3㎛ in the microstructure is 5% or less in terms of area ratio.
According to claim 3,
High-strength cold-rolled steel sheet, characterized in that the microstructure does not contain retained austenite.
According to claim 1 or 2,
The high-strength cold-rolled steel sheet, characterized in that the microstructure contains nano-precipitates of 10 nm or less at a distribution density of 150 / ㎛ ² or more.
According to claim 1 or 2,
The high-strength cold-rolled steel sheet is a high-strength cold-rolled steel sheet, characterized in that it has a tensile strength of 980 MPa or more, elongation of 14% or more, bending workability (R / t) of 1.0 or less, hole expandability (HER) of 40% or more.
C: 0.06-0.12%, Si: 0.5% or less, Mn: 2.0-3.0%, Cr: 0.3-1.2%, Ti: 0.02-0.08%, Nb: 0.01-0.06%, B: 0.001-0.005% by weight % Including, preparing a hot-rolled steel sheet by hot-rolling a steel base material consisting of the remaining Fe and unavoidable impurities;
cold-rolling the hot-rolled steel sheet at a reduction ratio of 55-70%;
After maintaining the cold-rolled steel sheet at a temperature that satisfies Equation 1 below, an annealing heat treatment step of cooling to a temperature of Ms or less; and
Including the step of overaging treatment of the annealed heat-treated steel sheet,
A high-strength cold-rolled steel sheet manufacturing method characterized in that it satisfies the following formula 1.
[Equation 1]
13≤0.8A-0.4(BC)≤27
A: Cold rolling reduction (%)
B: Ac3 temperature (℃)
C: annealing heat treatment temperature
According to claim 8,
The steel base material, in mass%, P: 0.001 ~ 0.10%, S: 0.010% or less, Sol.Al: 0.01 ~ 0.10% and N: high-strength cold-rolled steel sheet manufacturing method characterized in that it further comprises 0.01% or less.
delete According to claim 8,
High-strength cold-rolled steel sheet manufacturing method further comprising the step of skin pass rolling the cold-rolled steel sheet at a reduction ratio of 1% or less.
According to claim 8,
The high-strength cold-rolled steel sheet manufacturing method further comprising the step of forming a zinc plating layer or an alloy plating layer on the surface of the cold-rolled steel sheet. delete

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